GPS Displacement Research Articles

Determination of earthquake magnitude using GPS displacement waveforms from real-time precise point positioning

Determination of earthquake magnitude using GPS displacement waveforms from real-time precise point positioning

A high-resolution, time-variable afterslip model for the 2010 Maule Mw = 8.8, Chile megathrust earthquake

The excellent spatial coverage of continuous GPS stations in the region affected by the Maule Mw = 8.8 2010 earthquake, combined with the proximity of the coast to the seismogenic zone, allows us to model megathrust afterslip on the plate interface with unprecedented detail. We invert post-seismic observations from continuous GPS sites to derive a time-variable model of the first 420 d of afterslip. We also invert co-seismic GPS displacements to create a new co-seismic slip model. The afterslip pattern appears to be transient and non-stationary, with the cumulative afterslip pattern being formed from afterslip pulses. Changes in static stress on the plate interface from the co- and post-seismic slip cannot solely explain the aftershock patterns, suggesting that another process – perhaps fluid related – is controlling the lower magnitude aftershocks. We use aftershock data to quantify the seismic coupling distribution during the post-seismic phase. Comparison of the post-seismic behaviour to interseismic locking suggests that highly locked regions do not necessarily behave as rate-weakening in the post-seismic period. By comparing the inter-seismic locking, co-seismic slip, afterslip, and aftershocks we attempt to classify the heterogeneous frictional behaviour of the plate interface.

Application of real‐time GPS to earthquake early warning in subduction and strike‐slip environments

AbstractWe explore the application of GPS data to earthquake early warning and investigate whether the coseismic ground deformation can be used to provide fast and reliable magnitude estimations and ground shaking predictions. We use an algorithm to extract the permanent static offset from GPS displacement time series and invert for the slip distribution on the fault plane, which is discretized into a small number of rectangular patches. We developed a completely “self‐adapting” strategy in which the initial fault plane model is built based on a quick, approximate magnitude estimation and is then allowed to increase in size based on the evolutionary magnitude estimation resulting from the slip inversion. Two main early warning outputs are delivered in real‐time: magnitude and the along‐strike extent of the rupture area. These are finally used to predict the expected ground shaking due to the finite source. We tested the proposed strategy by simulating real‐time environments for three earthquakes. For theMw9.0, 2011 Tohoku‐Oki earthquake, our algorithm provides the first magnitude estimate of 8.2 at 39 s after the origin time and then gradually increases to 8.9 at 120 s. The estimated rupture length remains constant from the outset at ~360 km. For theMw8.3, 2003 Tokachi‐Oki earthquake, the initial magnitude estimate is 8.5 at 24 s and drops to 8.2 at 40 s with a rupture length of 290 km. Finally, for theMw7.2, 2010 El Mayor‐Cucapah earthquake, the magnitude estimate is 7.0 from the outset with a rupture length of 140 km. The accuracy of the ground shaking prediction using the GPS‐based magnitude and finite extent is significantly better than existing seismology‐based point source approaches. This approach would also facilitate more rapid tsunami warnings.

Repeated large Slow Slip Events at the southcentral Alaska subduction zone

We identify and study an ongoing Slow Slip Event (SSE) in the southcentral Alaska subduction zone using GPS measurements. This is the second large SSE in this region since modern geodetic measurements became available in 1993. We divide the ongoing SSE into two phases according to their transient displacement time evolution; their slip distributions are similar to each other but slip rates are slightly different. This ongoing SSE occurs downdip of the main asperity that ruptured in the 1964 Alaska earthquake, on the same part of the subduction interface as the earlier 1998–2001 SSE. The average slip rate of this SSE is ∼4–5cm/yr, with a cumulative moment magnitude of Mw 7.5 (Mw 7.3 and Mw 7.1 For Phases I and II, respectively) through the end of 2012. The time and space dependence of the GPS displacements suggest that the slip area remained nearly the same during Phase I, while the slip rate increased with time. The SSEs occur on a transitional section of the subduction plate interface between the fully locked updip part and the freely slipping deeper part. During the 1964 earthquake, slip on the region of the SSE was much lower than slip in the updip region. Based on this observation and the repeated SSEs, we conclude that this part of the interface slips repeatedly in SSEs throughout the interseismic period and does not build up a large slip deficit to be released through large slip in earthquakes.

Intense interface seismicity triggered by a shallow slow slip event in the Central Ecuador subduction zone

AbstractWe document a 1 week long slow slip event (SSE) with an equivalent moment magnitude of 6.0–6.3 which occurred in August 2010 below La Plata Island (Ecuador), south of the rupture area of the 1906 Mw = 8.8 megathrust earthquake. GPS data reveal that the SSE occurred at a depth of about 10 km, within the downdip part of a shallow (<15 km), isolated, locked patch along the subduction interface. The availability of both broadband seismometer and continuous geodetic station located at the La Plata Island, 10 km above the SSE, enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface. During the slow slip sequence, the seismic data show a sharp increase of the local seismicity, with more than 650 earthquakes detected, among which 50 have a moment magnitude between 1.8 and 4.1. However, the cumulative moment released through earthquakes accounts, at most, for 0.2% of the total moment release estimated from GPS displacements. Most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion. While the earthquake sizes show a classical distribution (Gutenberg‐Richter law with a b‐value close to 1), the space‐time occurrence presents a specific pattern. First, the largest earthquakes appear to occur randomly during the slow slip sequence, which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip. Moreover, the seismicity observed during the SSE consists in individual events and families of repeating earthquakes. These observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches. This study offers an a posteriori interpretation of the seismogenesis in the Central Ecuador subduction zone, where intense seismic swarms have been regularly observed (1977, 1998, 2002, and 2005). These swarms have likely been triggered by large‐magnitude slow slip events.

Numerical modelling of post-seismic rupture propagation after the Sumatra 26.12.2004 earthquake constrained by GRACE gravity data

In the last decades, the development of the surface and satellite geodetic and geophysical observations brought a new insights into the seismic cycle, documenting new features of inter-, co-, and post-seismic processes. In particular since 2002 satellite mission GRACE provides monthly models of the global gravity field with unprecedented accuracy showing temporal variations of the Earth's gravity field, including those caused by mass redistribution associated with earthquake processes. When combined with GPS measurements, these new data have allowed to assess the relative importance of afterslip and viscoelastic relaxation after the Sumatra 26.12.2004 earthquake. Indeed the observed post-seismic crustal displacements were fitted well by a viscoelastic relaxation model assuming Burgers body rheology for the asthenosphere (60–220 km deep) with a transient viscosity as low as 4 × 10^17 Pas and constant ~ 10^19 Pas steady state viscosity in the 60–660-km depth range. However, even the low-viscosity asthenosphere provides the amplitude of strain which gravity effect does not exceed 50 per cent of the GRACE gravity variations, thus additional localized slip of about 1 m was suggested at downdip extension of the coseismic rupture. Post-seismic slip at coseismic rupture or its downdip extension has been suggested by several authors but the mechanism of the post-seismic fault propagation has never been investigated numerically. Depth and size of localized slip area as well as rate and time decay during the post-seismic stage were either assigned a priory or estimated by fitting real geodesy or gravity data. In this paper we investigate post-seismic rupture propagation by modelling two consequent stages. First, we run a long-term, geodynamic simulation to self-consistently produce the initial stress and temperature distribution. At the second stage, we simulate a seismic cycle using results of the first step as initial conditions. The second short-term simulation involves three substeps, including additional stress accumulation after part of the subduction channel was locked; spontaneous coseismic slip; formation and development of damage zones producing afterslip. During the last substep post-seismic stress leads to gradual ~1 m slip localized at three faults around ~100-km downdip extension of the coseismic rupture. We used the displacement field caused by the slip to calculate pressure and density variations and to simulate gravity field variations. Wavelength of calculated gravity anomaly fits well to that of the real data and its amplitude provides about 60 per cent of the observed GRACE anomaly. Importantly, the surface displacements caused by the estimated afterslip are much smaller than those registered by GPS networks. As a result cumulative effect of Burgers rheology viscoelastic relaxation (which explains measured GPS displacements and about a half of gravity variations) plus post-seismic slip predicted by damage rheology model (which causes much smaller surface displacements but provides another half of the GRACE gravity variations) fits well to both sets of the real data. Hence, the presented numerical modelling based on damage rheology supports the process of post-seismic downdip rupture propagation previously hypothesized from the GRACE gravity data.

A Comparative Study on the System of Displacement Monitoring of Slope

At present, the research work of landslide monitoring is main focusing on how to adopt new technology and method to realize landslide displacement effective monitoring and data acquisition. Respectively introduces the principle of GPS displacement monitoring system, laser displacement monitoring system and line displacement monitoring system and for solving the problem. Through the technical and economic aspects of than to, it is concluded that the optimum scheme.

Permanent fore‐arc extension and seismic segmentation: Insights from the 2010 Maule earthquake, Chile

Geologists have long known that young normal faults are an important structural element of the Andean Coastal Cordillera, but their relationship to the subduction seismic cycle is still unclear. Some of the largest aftershocks of the 2010 Mw 8.8 Maule earthquake in central Chile were nucleated on upper plate normal faults, including the Mw 6.9 and 7.0 events of the Pichilemu earthquake sequence. We use the available coseismic GPS displacements, moment tensor sums, and slip distribution models for the Maule earthquake to compute the static strain and stress fields imposed on the upper plate by slip on the subduction interface. The extensional strains calculated from coseismic GPS and from a moment tensor sum of the Pichilemu events have similar orientations and orders of magnitude. The normal Coulomb stress increment (CSI) on the Pichilemu fault has maximum positive stresses as high as 4.9 MPa. Regionally, the Maule event produced a semi‐elliptical, radial pattern of static extension and deviatoric tension (CSI > 1.5 MPa) along the Coastal Cordillera enclosing the rupture area. This elliptical pattern mimics the trends of the major upper‐crustal structures. The static deformation field produced by a great subduction earthquake is an effective mechanism for generating permanent extension above the seismogenic zone, reactivating suitably oriented, long‐lived normal faults. We suggest that the semi‐elliptical outline of the first‐order structures along the Coastal Cordillera may define the location of a characteristic, long‐lived megathrust segment. This observation implies a persistence at least over the Quaternary of great subduction ruptures along the Maule segment.

Surface waves of the 2011 Tohoku earthquake: Observations of Taiwan's dense high‐rate GPS network

AbstractSurface waves generated by the 2011 Mw 9.0 Tohoku, Japan earthquake were recorded by both the high‐rate GPS and broadband seismic stations in Taiwan. In this study, we investigate the precision of high‐rate GPS displacements and estimate the feasibility of using GPS for seismology study in Taiwan. One hertz observations of 210 continuous GPS stations in Taiwan and neighboring islands were processed using a precise point positioning technique to estimate the absolute epoch‐by‐epoch positions generated by the Tohoku earthquake. Modified sidereal filtering was used to correct for near‐daily periodical variations of high‐rate position time series that may be influenced by multipath effects, and a band‐pass filter was used to remove noises from marginal frequencies. For all the 210 stations, the GPS precision after modified sidereal filtering improved from 8.2 to 7.4 mm in the horizontal and from 19.5 to 12.7 mm in the vertical components except for about 20% of the data with large position errors. After applying the band‐pass filter in the teleseismic frequency band (0.008–0.08 Hz), excluding 54 stations with noisy data, 156 continuous GPS stations were selected. Surface wave displacements derived from both high‐rate GPS and broadband seismometers are highly consistent, and the correlation coefficients are enhanced by a band‐pass filter (0.008–0.08 Hz) from 0.85 to 0.95 in the horizontal and from 0.58 to 0.85 in the vertical components. We consider continuous GPS can be used as an alternative approach to study characteristics of surface wave propagation in Taiwan.

Challenging the rate‐state asperity model: Afterslip following the 2011 M9 Tohoku‐oki, Japan, earthquake

Prior to the 2011 M9 Tohoku‐oki earthquake, subduction at the Japan Trench was characterized by M7‐8 earthquakes, sometimes rupturing the same source regions (seismic asperities), followed by extensive afterslip detected by GPS measurements. A physically‐based model consisting of velocity‐weakening asperities surrounded by aseismic creep on velocity‐strengthening regions (the ‘rate‐state asperity model’) became the prevailing conceptual model for earthquakes in this region. Theory and numerical simulation indicates that velocity‐weakening areas do not exhibit sustained afterslip, while velocity‐strengthening regions do not accumulate stress interseismically. Here we demonstrate that the rate‐state asperity model is contradicted by models of postseismic deformation following the Tohoku‐oki earthquake: afterslip in the first eight months either occurred on historical seismic asperities or stress accumulated in regions surrounding the asperities. Unsmoothed inversions of cumulative 8‐month postseismic GPS displacements that restrict afterslip to areas outside of historical ruptures cannot fit the data without afterslip exceeding the slip that fully relaxes the coseismic stress change. In contrast, similarly constrained inversions allowing slip within historical ruptures can satisfactorily fit the postseismic displacements. These results require a modification of the rate‐state asperity model and raise new questions about physical processes and properties of the subduction interface.

The Western Crete geodetic infrastructure: Long-range power-law correlations in GPS time series using Detrended Fluctuation Analysis

The Global Positioning System makes it possible, nowadays, to measure crustal displacements with unprecedented accuracy. These measurements can improve our understanding of the spatio-temporal evolution of motion along tectonic plate boundaries, as well as deepen our comprehension for the nature of earthquake fault behavior and earthquake cycle. The Hellenic subduction zone and the earthquake-prone Crete, with large crustal motions of 3–4cm/yr, may provide such a natural laboratory for this type of investigation. In this work, we examine the statistical structure and behavior of time series, as produced by permanent GPS sites, established on the islands of Crete and Gavdos.At first, we investigate the crustal motion based on daily displacements as monitored by a regional GPS network, established on either sides of faults, in Gavdos and Crete. Secondly, we reconstruct the missing data in the time series of two GPS stations, which contain the longest observations, by selecting an appropriate autoregressive stochastic model. This is accomplished by representing the state-space behavior of the signal using a Kalman filter. Finally, the Detrended Fluctuation Analysis has been applied on the residuals for the North and East components of the GPS displacements. In conclusion, there seems to exist strong evidence, at least for the period of measurements of 7.5years, that both investigated stations show similar statistical behavior in the form of power law and exhibit the same (∼0.9–1.0) long-range correlation.

5Hz GPS seismology of the El Mayor-Cucapah earthquake: estimating the earthquake focal mechanism

High-rate (5Hz) GPS records observed in the near field following the magnitude 7.2 El Mayor—Cucapah earthquake that occurred in northern Mexico on 2010 April 4 are compared with broad-band seismograms. The high-rate GPS displacement records are consistent with the twice-integrated strong-motion seismic records in the near field where broad-band seismograms are clipped due to strong shaking. Agreement degrades at distances greater than about 150 km from the epicentre where displacement amplitudes approach the noise level of GPS seismograms. Using high-rate GPS data the focal mechanism of the main shock is estimated and is shown to be consistent with teleseismic estimates. The result is seen as confirmation that high-rate GPS observed at near-field stations can be applied together with teleseismic seismometers to yield better information about earthquake rupture properties and parameters.

Slip distribution of the 2011 Tohoku earthquake derived from joint inversion of GPS, InSAR and seafloor GPS/acoustic measurements

We invert the fault slips of 2011 Mw 9.0 Tohoku earthquake with constraints from GPS, InSAR and seafloor GPS/acoustic measurements. The seismogenic fault geometry is constructed according to slab contours of Japan Trench. Steepest Descent method and Laplacian smoothing are used to solve slip distribution and regularize the solution. We firstly take GPS displacement in two postseismic periods, the first 8h right after mainshock from 5:55 to 14:00 UTC and the 13days from 12 March 2011 to 25 March 2011, to solve for the postseismic slips. The solved postseismic slips are adopted to remove postseismic signal in InSAR and seafloor observation. In order to estimate the effect of postseismic correction and contribution from different geodetic datasets, we invert several coseismic slips with constraints from GPS (Model 1), corrected InSAR (Model 2), combination of GPS and corrected seafloor measurements (Model 3), combination of GPS and corrected InSAR and seafloor measurements (Model 4), and combination of GPS and initial InSAR and seafloor measurements (Model 5). From the comparison of these slip models, we find combined datasets could give more slip details, which is closer to a joint inversion result constrained from both seismic and geodetic datasets (Koketsu et al., 2011). RMSE of seafloor measurements has dropped about 4cm after applying postseismic correction. We consider the Model 4, which combines three datasets and takes postseismic correction, to be the preferred solution among all the estimated models. It suggests a maximum slip of 49.87m, located at a depth of 5km around the epicenter, and has a geodetic moment of 3.14×1022Nm (Mw 8.96) by assuming a shear modulus of 4×1010Pa.

The 2010 Mw 7.8 Mentawai earthquake: Very shallow source of a rare tsunami earthquake determined from tsunami field survey and near‐field GPS data

The Mw 7.8 October 2010 Mentawai, Indonesia, earthquake was a “tsunami earthquake,” a rare type of earthquake that generates a tsunami much larger than expected based on the seismic magnitude. It produced a locally devastating tsunami, with runup commonly in excess of 6 m. We examine this event using a combination of high‐rate GPS data, from instruments located on the nearby islands, and a tsunami field survey. The GPS displacement time series are deficient in high‐frequency energy, and show small coseismic displacements (<22 cm horizontal and <4 cm subsidence). The field survey shows that maximum tsunami runup was >16 m. Our modeling results show that the combination of the small GPS displacements and large tsunami can only be explained by high fault slip at very shallow depths, far from the islands and close to the oceanic trench. Inelastic uplift of trench sediments likely contributed to the size of the tsunami. Recent results for the 2011 Mw 9.0 Tohoko‐Oki earthquake have also shown shallow fault slip, but the results from our study, which involves a smaller earthquake, provide much stronger constraints on how shallow the rupture can be, with the majority of slip for the Mentawai earthquake occurring at depths of <6 km. This result challenges the conventional wisdom that the shallow tips of subduction megathrusts are aseismic, and therefore raises important questions both about the mechanical properties of the shallow fault zone and the potential seismic and tsunami hazard of this shallow region.

Estimating coseismic coastal uplift with an intertidal mussel: calibration for the 2010 Maule Chile earthquake (Mw = 8.8)

Coseismic coastal uplift has been quantified using sessile intertidal organisms after several great earthquakes following FitzRoy's pioneer measurements in 1835. A dense survey of such markers may complement space geodetic data to obtain an accurate distribution of fault slip and earthquake segmentation. However, uplift estimates based on diverse intertidal organisms tend to differ, because of few methodological and comparative studies. Here, we calibrate and estimate coastal uplift in the southern segment of the 2010 Maule, Chile earthquake (Mw = 8.8) using >1100 post-earthquake elevation measurements of the sessile mussel Perumytilus purpuratus. This mussel is the predominant competitor for rocky shores all along the Pacific coast of South America, where it forms fringes or belts distinctively in the middle intertidal zone. These belts are centered at mean sea level and their width should equal one third of the tidal range. We measured belt widths close to this value at 40% of the sites, but overall widths are highly variable due to the unevenness in belt tops; belt bases, in turn, are rather regular. Belt top unevenness apparently results from locally-enhanced wave splash, whereas belt base evenness is controlled by predation. According to our measurements made beyond the earthquake rupture, the belt base is at the bottom of the middle intertidal zone, and thus we propose to estimate coastal uplift using the belt base mean elevation plus one sixth of the tidal range to reach mean sea level. Within errors our estimates agree with GPS displacements but differ from other methods. Comparisons of joint inversions for megathrust slip suggest combining space geodetic data with estimates from intertidal organisms may locally increase the detail of slip distributions.

Quasi-synchronous multi-parameter anomalies associated with the 2010–2011 New Zealand earthquake sequence

Abstract. Positive thermal anomalies about one month before the 3 September 2010 Mw = 7.1 New Zealand earthquake and "coincidental" quasi-synchronous fluctuations of GPS displacement were reported. Whether there were similar phenomena associated with the aftershocks? To answer it, the following was investigated: multiple parameters including surface and near-surface air temperature, surface latent heat flux, GPS displacement and soil moisture, using a long-term statistical analysis method. We found that local thermal and deformation anomalies appeared quasi-synchronously in three particular tectonic zones, not only about one month before the mainshock, but also tens of days before the 21 February 2011 Mw = 6.3 aftershock, and that the time series of soil moisture on the epicenter pixel had obvious peaks on most of the anomalous days. Based on local tectonic geology, hydrology and meteorology, the particular lithosphere-coversphere-atmosphere coupling mode is interpreted and four mechanisms (magmatic-hydrothermal fluids upwelling, soil moisture increasing, underground pore gases leaking, and positive holes activating and recombining) are discussed.

Inference of coseismic slip via joint inversion of GPS and aftershock data: The 2004 Parkfield example

In many coseismic slip inversions, the number of model parameters is much larger than that of the independent observations, and the problem is extremely underdetermined. It is thus instructive to incorporate additional data sets into the slip inversion. In this study I describe a new approach for coseismic slip inversion, whereby both GPS displacements and first day aftershock rate changes are used jointly to constrain the solution. The joint inversion incorporates the Dieterich's aftershock model, which adopts a constitutive friction that depends logarithmically on the sliding rate. The method is applied to the 2004 Parkfield earthquake. The joint inversion not only provides resolving power of slip at depths inaccessible to GPS‐only inversions, but it also helps to gain insight on the fault mechanical properties. I show that the data are consistent with the adopted aftershock model being the dominant mechanism for aftershock production along the Parkfield segment, and I obtain an upper bound on the friction dependence on the log of rate of fault patches that have experienced aftershock activity. A consequence of the irregular aftershock distribution is that the slip distribution is extremely nonsmooth, with the aftershock zones acting as barriers.

The Mw 9.0 northeast Japan earthquake: total rupture of a basement asperity

SUMMARY The Mw 9.0 off northeast Japan (Tohoku-oki) earthquake occurred on 2011 March 11 at the interface between the North American (NA) and Pacific (PA) plates. The occurrence of interplate earthquakes can be regarded as the sudden release of tectonic stress accumulated by the interseismic gradual increase of slip deficit in source regions. The inversion analysis of GPS velocity data for a seismically calm period (1996–2000) before the Tohoku-oki earthquake has revealed the existence of five remarkable slip-deficit zones distributed on the NA–PA plate interface along the southern Kuril-Japan trench. We analysed coseismic GPS displacement data for the Tohoku-oki earthquake with the same inversion method, and obtained the bimodal distribution of coseismic slip spreading over the southern two, Miyagi-oki and Fukushima-oki, interseismic slip-deficit zones. The maximum slip is 32 m for the Miyagi-oki slip-deficit zone but only 7 m for the Fukushima-oki slip-deficit zone. The extraordinarily large coseismic slip in the Miyagi-oki slip-deficit zone, where ordinarily large earthquakes with about 3 m coseismic slips have repeated every 40 yr in the past two centuries, suggests the total rupture of a 300-km-long basement asperity underlying much smaller-scale local asperities.

Effects of Correction of Atomospheric Delays on GPS Displacement Measurements in Large Slopes

Effects of Correction of Atomospheric Delays on GPS Displacement Measurements in Large Slopes

Real-time centroid moment tensor determination for large earthquakes from local and regional displacement records

SUMMARY We present an algorithm to rapidly determine the moment tensor and centroid location for large earthquakes employing local and regional real-time high-rate displacement records from GPS. The algorithm extracts the coseismic offset from the displacement waveforms and uses the information to invert for the moment tensor. The Green's functions for a layered earth are obtained numerically from open source code EDGRN. To determine the centroid, multiple inversions are simultaneously performed within a grid of inversion nodes, and the node with the smallest misfit is then assigned the centroid location. We show results for two large earthquakes replayed in simulated real-time mode using recorded 1 Hz GPS displacements: the 2003 Mw 8.3 Tokachi-oki and the 2010 Mw 7.2 El Mayor-Cucapah earthquakes. We demonstrate that it is feasible to obtain accurate CMT solutions within the first 2–3 min after rupture initiation without any prior assumptions on fault characteristics, demonstrating an order of magnitude improvement in latency compared to existing seismic methods for the two earthquakes studied. This methodology is useful for rapid earthquake response, tsunami prediction and as a starting point for rapid finite fault modelling.